The present invention relates to imide-based prepolymers having high heat stability. In particularly, the present invention relates to a process for producing imide-based prepolymers having high solvent solubility and to a varnish using such prepolymers.
A conventional process for producing imide-based prepolymers, in particular, those having good heat stability is described in Japanese Patent Publication No. 46-23250; a bisimide is mixed with a diamine under heating and the reaction occurs in a molten state, or a bisimide is reacted with a diamine under heating in a strongly polar and high-boiling point solvent such as N-methylpyrrolidone or dimethylformamide.
The imide-based resin produced by the method described in Japanese Patent Publication No. 46-23250 dissolves only in strongly polar and high-boiling point solvents such as N-methylpyrrolidone and dimethylformamide and the varnish prepared from such imide-based prepolymers does not have high storage stability.
This varnish may be impregnated in a glass cloth to prepare a prepreg and a plurality of such prepregs are superposed with copper foils to make a laminated board. However, because of its high boiling point, the solvent for the varnish will remain in the laminate, thus reducing the peel strength of a copper foil in the laminated board. This has been on of the major causes of deterioration in the quality of laminated boards.